Spring Constants of Belled Caissons for Seismic Retrofit of an Existing Critical Structure in a Seismically Active Area-Paper

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The paper presents a method for estimating foundation spring constants and capacities for individual belled caissons supporting an existing hospital in Long Beach, California. The seismic retrofit evaluation required these parameters for the BSE-1E and BSE-2E performance levels, but ASCE 41-13 did not directly address the axial behavior of belled caissons or the rocking stiffness of single caissons.<br /><br />For axial stiffness, each caisson was modeled as bell, shaft, and pile cap components in series. Their structural stiffness was combined with the stiffness of the underlying soil, estimated using shallow-footing relationships and a Winkler-type model. Lateral stiffness was calculated using NIST pile-foundation relationships, with the caisson shaft represented as a pile. Rocking stiffness was estimated using lateral-foundation analysis software by relating applied head moment to rotation.<br /><br />Because most caisson bases had depth-to-diameter ratios suitable for shallow-foundation methods, those methods were also used to estimate compression capacity. Uplift capacity was calculated using relationships from Poulos and Davis. Lateral capacity was not calculated in advance because it depends on the applied shear and moment; it can be checked after structural analysis.<br /><br />The authors provide stiffness and capacity estimates for 23 caisson configurations at both seismic hazard levels. To address uncertainty in soil properties and foundation behavior, they recommend evaluating upper- and lower-bound values, using a factor of two where detailed testing is unavailable. Overall, the proposed approach adapts existing methods to belled, single-element caissons not explicitly covered by the cited code provisions.

Keywords

belled caisson

foundation stiffness

seismic retrofit

axial stiffness

lateral stiffness

rocking stiffness

foundation capacity

ASCE 41-13

BSE-1E

BSE-2E